Heavy Metal! - March - 2021

Diagnosing a Subaru Outback AWD

It never fails; no matter how long you have been in the business, you’ll always run into that one vehicle that will leave you standing there scratching your head. I received a call from my good friends Steve Wilson and Junior from Gold Coast Transmission in Hollywood, FL. There was a 2006 Subaru Outback AWD with the 5EAT and 72,289 miles on it in their shop that they needed help with.

When Steve and Junior road-tested this vehicle, they could feel a shudder/ vibration under hard acceleration. The usual tight corner thumping was there as well, meaning there was more than one issue.

The car drove perfectly fine after takeoff, even under a heavy throttle upshift. But when you released the gas pedal, there was a clunk. Besides those issues, the shift scheduling and shift feel were right on point.

A common problem with these transmissions is the transfer clutches and, since the clunk wasn’t too apparent, they wanted to address those clutches first. The problem there is usually caused by high mileage or the wrong fluid.

The AWD transfer clutches use a hydraulically-control multi-plate clutch. This clutch is controlled by the TCM through a control valve. The TCM uses programmed-duty-ratio-data to define the ratio and the torque for a particular driving situation. The TCM controls the clutch apply, depending on engine torque, throttle opening, vehicle speed, gear range, and wheel rpm’s through the ABS sensors.

Torque is delivered to the rear wheels through the transfer clutch and into the rear differential (depending on driving conditions). Note: One thing that can be done during diagnosis is to disable the AWD application to determine a possible issue with the transfer clutch. With the AWD off and out of the way, we can concentrate on other components.

They removed the transmission, and sure enough, the transfer clutch had failed. After an overhaul and test drive, the clunking noise was still there, but it was now obvious it wasn’t in the transmission.

The key to all diagnostics is knowing how the systems you’re working on operate. This is the first rule of engagement when it comes to diagnosing anything.

Once we got the vehicle back from the test drive, it was obvious that the rear differential was making the noise. Take a good look at Figure one and look at the spider gear’s pin. As you can see, this pin has been hammering the carrier for quite a while. Being that the AWD system is on sometimes, there wasn’t enough torque to the rear end to make significant noise. That is until we were test driving with the AWD system enabled. The differential is supposed to look something like this (Figure 2). So now we had a failed differential to address, too (but at least we found it).

Like I mentioned before, the vehicle was driving normally except for the shudder under hard acceleration during takeoff. The only abnormal PID in the scanner was the OSS at the AWD section. Anytime you went into passing gear or accelerated hard, the sensor would act up. The sensor will also act up on the highway, usually in 5th gear.

The problem with the AWD system made the differential failure more difficult to notice. In the end, both problems needed fixing, but it’s another lesson on how we might miss part of the customer’s complaint if we’re not careful.

Till next time Cya.